With the modern development of information technologies, two-way communications via a network are activated more and more. In two-way communications, some use a communication system based on Voice over Internet Protocol (VoIP) that enables voice calls via the Internet by sending/receiving packetized voice data via a communication network.
As the volume of IP telephone calls transacted via a packet communication network increases, the IP phone providers, etc., put more emphasis on monitoring the quality of telephone calls. In order to identify a point at which a failure occurs in a communication system, a monitoring device is installed to monitor the quality of network communications. More particularly, a monitoring device calculates, for example, a disappearance rate of packets flowing in a packet communication network and a delay of packets, thereby measuring the communication quality and then identifies a point at which a failure occurs during a monitoring process.
Positions where monitoring devices are installed are described with reference to FIG. 17. As illustrated in FIG. 17, a communication system includes a sending device that sends packets, a relay device that relays packets, monitoring devices that monitor packets, and a receiving device that receives packets. In the example of FIG. 17, the system has two sections: a first section covers from the sending device to the relay device, and a second section covers the relay section to the receiving device. The point that divides the first section from the second section is called “dividing point”. A monitoring device is located in the first section and monitors the first section; and another monitoring device is located in the second section and monitors the second section.
In the above system, the monitoring device located in the first section measures the communication quality of packets coming from the sending device and monitors whether a failure occurs in the first section. The monitoring device located in the second section measures the communication quality of forward packets coming from the relay device and monitors whether a failure occurs in the second section (more accurately, area from the dividing point to the second monitoring device). The relay device assigns a sequence number to a packet coming from the sending device and then forwards the packet to the receiving device.
Patent Document 1: Japanese Laid-open Patent Publication No. 2006-108834
Patent Document 2: Japanese Laid-open Patent Publication No. 2009-94877
Patent Document 3: Japanese National Publication of International Patent Application No. 2003-500933
In the above packet-monitoring communication system, in order to identify a failure as either a failure that occurs in the first section or a failure that occurs in the second section, a monitoring device is located on each section. This limits positions where monitoring devices can be installed and increases the number of monitoring devices installed and thus increases the costs for installation of monitoring devices.